Gammaherpesviruses infect a majority of humans and are associated with cancer in susceptible populations, including HIV/AIDS patients. A better understanding of the mechanism whereby the host restricts chronic infection is likely to stimulate the development of new therapeutic approaches aimed at decreasing the risk of virus-driven cancer. Here we propose that Ataxia-Telangiectasia mutated (ATM) kinase is an important host factor that regulates chronic gammaherpesvirus infection. We hypothesize that ATM expression by T cells is required for the development of an optimal gammaherpesvirus-specific adaptive immune response. In parallel, ATM is usurped in infected cells to facilitate vira reactivation. These opposing functions establish a virus-host balance that, when perturbed, alters parameters of chronic infection and viral pathogenesis. This hypothesis is supported by our published and preliminary studies and clinical observations indicating that ATM insufficient humans are selectively susceptible to severe herpesvirus infection. The hypothesis will be tested by specific aims that are expected to 1) determine the molecular mechanism by which ATM facilitates gammaherpesvirus reactivation; 2) determine the mechanism by which ATM supports the development of virus-specific adaptive immune response, and 3) determine the contribution of ATM to the maintenance of infected cell reservoir during long-term infection. Successful completion of the proposed studies will allow a better insight into the regulation of chronic gammaherpesvirus infection by ATM, an insight that we hope will stimulate new therapeutic approaches aimed to control gammaherpesvirus infection and pathogenesis in at-risk populations, including HIV patients.